In transmitting signals, prior art radio systems use various diversity methods to improve the quality of data transmission. Diversity methods include, for instance, orthogonal transmit diversity (OTD), time switching transmit diversity (TSTD) and selective transmit diversity (STD). The above-mentioned diversity methods can be used in future WCDMA systems, for instance. Said methods improve BER performance, for instance, in data transmission. Of the above-mentioned methods, especially the STD method provides the greatest advantages as compared with the OTD and TSTD methods, for instance.
In FDD systems, the STD method can be used in selecting antennas, for instance. In this case, in a radio system, a subscriber terminal, which can be a mobile phone, for instance, selects and advises the base station to select an as optimal antenna as possible for the base station to use in a downlink signal transmission. The selection of antennas is based on measuring the quality of signals transmitted by the base station antennas and comparing the obtained measurement results with each other.
However, the use of STD methods causes load problems in the power amplifiers of the transceiver. The problems are caused because the load of the power amplifiers is not always distributed evenly between the various power amplifiers, and the differences in loads may be relatively big. In a CDMA-type radio system, for instance, a situation may arise in practice where a given transmission branch of a base station is selected to transmit signals to a large number of subscriber terminals which establish simultaneous connections by means of the signals. Such a selection method calls for extensive dynamics of the power amplifier at the transmission branch. Extensive dynamics call for using a high crest coefficient in designing the power amplifiers, which coefficient is determined by the ratio of the maximum output required of the power amplifier and its average output.
Let us assume that the base station comprises a first and a second transmission antenna which transmit a signal to the same subscriber terminal. Let us further assume that the base station comprises a first power amplifier feeding its signal to the first transmission antenna and a second power amplifier feeding its signal to the second transmission antenna. If the subscriber terminal is advantageously located with respect to the first transmission antenna, for instance, the subscriber terminal receives a signal of at least slightly better quality from the first transmission antenna. In practice, it may well be possible that the signal transmitted by the second antenna is also received as a relatively good-quality signal. If the antenna is selected to transmit a signal to mobile phones, of which there are K and to each of which the base station power amplifier transmits the signal at P power, the dynamics of the base station power amplifier must achieve a power level of at least KP.
Because the selection of antennas is based solely on measuring the absolute quality of the signal, the subscriber terminal sends to the base station a command to use the first transmission antenna. If sufficiently many subscriber terminals command the base station to use the first transmission antenna, it is possible that the nominal loadability of the first power amplifier is exceeded. If the subscriber terminal has had to select the better of two base station antennas, for instance, the subscriber terminal can transmit information on the selection by using one selection bit. In the above-mentioned situation, the value of the selection bit may be ‘1’, for instance, indicating the selection of the first transmission antenna, for instance. The value ‘0’ of the selection bit would then have indicated the selection of the second transmission antenna.
The selection method is, however, not optimal from the point of view of the power amplifier load, since the used method may lead to overload in the power amplifier feeding the selected transmission antenna. The load of the power amplifiers is thus too uneven in some situations. Said problems are at least partly caused by the fact that measuring the signal quality is based on absolute values which does not lead to the best possible solution for the operation of the system.